Tungsten nitride is used in several applications for semiconductor device fabrication. As deposited by traditional means such as PVD and PECVD, tungsten nitride provides relatively low resistivity, good adhesion to dielectric films, and is a good diffusion barrier. A key limitation that has prevented wider application of WN in the past has been poor step coverage in high aspect ratio trenches, vias and contacts.
To be successful in nanometer scale applications, tungsten nitride must be deposited thinly and conformally in high aspect ratio features. Conventional physical vapor deposition (PVD) techniques are not able to meet these criteria. To accomplish thin conformal coverage, chemical vapor deposition (CVD) methods are typically considered. A conventional CVD process involves the simultaneous introduction of gas phase reactants, including tungsten precursor (typically tungsten hexafluoride (WF6)) and a nitrogen containing gas (e.g., N2), near a heated wafer surface while a vacuum is applied to the system. The reaction is driven by the energy provided by the heated wafer and the free energy change of the chemical reaction. The growth of the tungsten nitride film continues as long as the reactants and energy source are available.
Although standard tungsten nitride CVD techniques can provide good step coverage and adequately fill low aspect ratio features (e.g., <5:1 aspect ratio), as semiconductor fabrication technology approaches the nanometer scale, the demands for step coverage and gap filling are becoming more stringent and CVD may not be suitable for the task. Traditional plasma-enhanced CVD of tungsten nitride has relatively poor step coverage for a CVD process (<50% SC in a 5:1 aspect ratio cylindrical contact). This is not adequate for the demands of current and future semiconductor technology with aspect ratios exceeding 10 to 1 and critical dimensions less than 100 nanometers. CVD and particularly PNL or ALD tungsten processes (as opposed to tungsten nitride processes) can provide the very high step coverage and conformal deposition required for modern semiconductor devices, but will not adhere directly to dielectric surfaces. Tungsten now requires an adhesion layer such as TiN before deposition on dielectric surfaces. Finally, the high deposition temperatures required by many TiN deposition techniques (e.g. PECVD-TiN from TiCl4) require high deposition temperatures that are incompatible with low-K dielectrics or nickel silicide.
What are therefore needed are improved methods for depositing tungsten nitride.